Autoregulation of glucose production in men with a glycerol load during rest and exercise

Related to hepatic autoregulation we evaluated hypotheses that 1) glucose p roduction would be altered as a result of a glycerol load, 2) decreased glu cose recycling rate (Rr) would result from increased glycerol uptake, and 3 ) the absolute rate of gluconeogenesis (GNG) from glycerol would be positiv ely correlated to glycerol rate of disappearance (R-d) during a glycerol lo ad. For these purposes, glucose and glycerol kinetics were determined in ei ght men during rest and during 90 min of leg cycle ergometry at 45 and 65% of peak O-2 consumption ((V)over dotO(2) peak). Trials were conducted after an overnight fast, with exercise commencing 12 h after the last meal. Subj ects received a continuous infusion of [6,6-H-2(2)]glucose, [1-C-13] glucos e, and [1,1,2,3,3-H-2(5)]glycerol without (CON) or with an additional 1,000 mg (rest: 20 mg/min; exercise: 40 mg/min) of [2-C-13]- or unlabeled glycer ol added to the infusate (GLY). Infusion of glycerol dampened glucose Rr, c alculated as the difference between [6,6-H-2(2)]- and [1-C-13] glucose rate s of appearance (R-a), at rest [0.35 +/- 0.12 (CON) vs. 0.12 +/- 0.10 mg.kg (-1).min(-1) (GLY), P < 0.05] and during exercise at both intensities [45%: 0.63 <plus/minus> 0.14 (CON) vs. 0.04 +/- 0.12 (GLY); 65%: 0.73 +/- 0.14 ( CON) vs. 0.04 +/- 0.17 mg.kg(-1).min(-1) (GLY), P < 0.05]. Glucose R-a and oxidation were not affected by glycerol infusion at rest or during exercise . Throughout rest and both exercise intensities, glycerol R-d was greater i n GLY vs. CON conditions (rest: 0.30 <plus/minus> 0.04 vs. 0.58 +/- 0.04; 4 5%: 0.57 +/- 0.07 vs. 1.19 +/- 0.04; 65%: 0.73 +/- 0.06 vs. 1.27 +/- 0.05 m g.kg(-1).min(-1), CON vs. GLY, respectively). Differences in glycerol R-d ( DeltaR(d)) between protocols equaled the unlabeled glycerol infusion rate a nd correlated with plasma glycerol concentration (r = 0.97). We conclude th at infusion of a glycerol load during rest and exercise at 45 and 65% of (V )over dotO(2 peak) 1) does not affect glucose R-a or R-d, 2) blocks glucose Rr, 3) increases whole body glycerol R-d in a dose- dependent manner, and 4) results in gluconeogenic rates from glycerol equivalent to CON glucose r ecycling rates.